Threaded fasteners have long provided a convenient, efficient and secure method of fastening parts together. Typically, a male threaded fastener, consisting of an elongated cylindrical shank having helical threads formed thereabout, is driven through a nut or hole commonly having threads formed therein. The nut typically has a flange extending from the interiorly-threaded hole. This nut flange cooperates with a flange or head on the end of the male threaded fastener's shank, so as to hold together material through which the male threaded fastener is inserted. Such fastening arrangements have been so long known in the art that their origin is probably unascertainable. Nonetheless, new problems are constantly being faced by the threaded fastener industry, and adaptations of the basic threaded elemental design are being discovered to solve the unique problems which seemingly endlessly present themselves to the industry.
Characteristically, whenever a male threaded fastener, or screw is inserted into a threaded, female threaded fastener, or nut, the surfacial contact between the helical screw thread and the helical nut thread is not normally 100%. Hereafter, in this description, the term "screw" will be used interchangeably with, and will be used to mean the same as the male threaded fastener having helical threads formed thereabout. Hereafter also, the term "nut" shall be used to describe the female threaded fastener. The term "nut" could be used interchangeably with the term female threaded fastener. The term "nut", also, shall be used hereafter to define any hole through which the screw may be inserted and secured thereto to form a fastening.
Examples of past improvements and variations of the basic threaded fastener screw and nut design can be seen in U.S. Pat. No. 271,852 to Hull and U.S. Pat. No. 597,000 to Higbee. In Higbee, it can be particularly appreciated, partially formed threads on the screw or male fitted fastener member are reformed when inserted or forcefully driven into a perfectly formed threaded nut or hole. Such teachings, however, do not appreciate the particular problems encountered by current industrial techniques, inasmuch as it is the forming of threads within an internal hole or nut which present unique production problems and opportunities for savings and efficiency.
The term "thread-to-thread engagement" hereafter will be used to denominate or to define the ratio of (1) the surface area of the helical screw threads which actually contact the surface area of the helical nut threads, when compared with (2) the total surface area of the helical screw threads within the hole of the nut. The imperfect, less than 100% surfacial mating of the screw threads with the nut threads, or thread-to-thread engagement can be caused by the fact that the helical pitch of the screw could be slightly varied from the helical pitch of the threads in the nut. Moreover, the pitch angle of the screw could vary slightly from the pitch angle of the nut. Also, mere imperfections or deviations in the molding, casting or rolling of the threads of the screw could make its threads vary from the formed threads of the nut. Thus, on one, two or several of the helical threads of the screw, an air pocket hole, lump or manufacturing crease or tail might be found. Moreover, the screw thread might have some of its turns flattened or otherwise deformed. Additionally, temperature may change the precise cross-sectional configuration of the screw's threads. Likewise on the nut thread, temperature, handling and manufacturing factors may cause one, two or more of the helical nut threads to be deformed, or to otherwise vary from the desired or designed thread form. Thus, when a screw is inserted into a nut, it is uncommon to have greater than 55% thread-to-thread engagement.
It has been found that thread-to-thread engagement exceeding 60% does not improve the shearing strength of the threads, but does improve the rotational friction resistance between the nut and the bolt. Such friction resistance is commonly referred to as prevailing torque. The greater the prevailing torque, it can be appreciated, the lesser the tendency for the bolt to loosen from the nut in vibrational and load stresses. Thus in the industry, a threaded fastener arrangement having a greater prevailing torque is said to have a greater locking ability. Thread-to-thread engagement of greater than 60%, however, does not necessarily increase the static strength of the threaded fastener, inasmuch as the shearing strength remains constant unless there has been some stripping or shaving of the bolt or male thread. The total surface area of the screw thread in the context used here, shall mean only the total surface area of the screw thread which at any one time comes within the boundaries of the nut's threaded area.
Frequently the outside and inside diameters of the nut threads are purposely formed to be larger than the outside and inside diameters of the screw threads, so that when the screw is inserted into the nut there will be no possibility that the screw will be forced to form or to "push steel". The term to "push steel" is commonly used within the thread fastener industry to describe the function a screw performs when it is inserted into a smaller nut hole, and is thereby obliged in the forming of the nut thread to remove steel from the nut hole, and to push it ahead of the bolt through the hole. This interference of the bolt with the nut when the nut and the bolt are of like material or when the bolt is of a harder material than the nut or female thread, results in a galling and stripping away of portions of the bolt or male thread. In such circumstances it can be appreciated, the male thread would later have a greater tendency to fail since its original thread root thickness and design has been substantially reduced. S.A.E. and most industrial specifications require the bolt or male thread tolerance to exceed twice the nut or female thread tolerance. This requirement is a result of such galling or stripping of the male thread. Moreover, such a designed greater outside and inner diameters of the male nut thread has a tendency to result in the lessening of the thread-to-thread engagement ratio.
It is desired to increase the thread-to-thread engagement ratio without requiring the screw so to push steel.